1. Field
The present invention relates generally to communication, and more specifically to data transmission in a multiple-input multiple-output (MIMO) communication system that utilizes orthogonal frequency division multiplexing (OFDM).
2. Background
A MIMO system employs multiple (T) transmit antennas at a transmitting entity and multiple (R) receive antennas at a receiving entity for data transmission. A MIMO channel formed by the T transmit antennas and R receive antennas may be decomposed into S spatial channels, where S≦min {T, R}. The S spatial channels may be used to transmit data in parallel to achieve higher throughput and/or redundantly to achieve greater reliability.
OFDM is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (K) orthogonal frequency subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data.
A MIMO-OFDM system is a MIMO system that utilizes OFDM. The MIMO-OFDM system has S spatial channels for each of the K subbands. Each spatial channel of each subband may be called a “transmission channel”. Each transmission channel may experience various deleterious channel conditions such as, e.g., fading, multipath, and interference effects. The transmission channels for the MIMO channel may also experience different channel conditions and may achieve different signal-to-noise-and-interference ratios (SNRs). The SNR of each transmission channel determines its transmission capacity, which is typically quantified by a particular data rate that may be reliably transmitted on the transmission channel. For a time variant wireless channel, the channel conditions change over time and the SNR of each transmission channel also changes over time. The different SNRs for different transmission channels plus the time varying nature of the SNR for each transmission channel make it challenging to efficiently transmit data in a MIMO system.
If the transmitting entity has knowledge of the channel condition, then it may transmit data in a manner to more fully utilize the capacity of each transmission channel. However, if the transmitting entity does not know the channel condition, then it may need to transmit data at a low rate so that the data transmission can be reliably decoded by the receiving entity even with the worst-case channel condition. Performance would then be dictated by the expected worst-case channel condition, which is highly undesirable.
There is therefore a need in the art for techniques to more efficiently transmit data in a MIMO-OFDM system, especially when the channel condition is not known by the transmitting entity.